A plasma cutting machine comprises an electrode and a nozzle, wherein the nozzle provides a path between the nozzle and the electrode through which a working gas can flow and be directed toward the object to be cut. While the working gas is being supplied, a pilot arc is initially generated between the electrode and the nozzle, and then a main arc, which is a hot plasma arc, is generated between the electrode and the object to be cut, thereby cutting the object.
A known technique to control a plasma arc cutting machine will be described first. In the following description, it will be assumed that gaseous oxygen is used as the working gas. FIG. 6 illustrates a configuration of a typical conventional plasma arc cutting machine. FIG. 7 is a timing chart showing the sequence of operations. In response to a start signal S.sub.T inputted to the plasma arc cutting machine, a constant current source 8 is turned "on" and a switch 10 is closed, whereby a negative DC voltage is applied to an electrode 1a in a plasma torch 1, and a positive DC voltage is applied to the nozzle 1b of the plasma torch 1 and to a workpiece 11 to be cut. At the same time, a stop valve 4 is opened and a preflow of gaseous oxygen is supplied from a gas source to the inside of the plasma torch 1 via a gas flow line 2 containing a pressure regulator 3 and the stop valve 4. The preflow is supplied for a suitable period of time so as to replace any air in the gas supply pipe 2 with oxygen. This preflow period also provides time for the gas flow to settle to a stable state.
After the preflow period, a high frequency generator 9 is turned on, and a high frequency high voltage is applied between the electrode 1a and the nozzle 1b, whereby a spark discharge occurs between the electrode 1a and the nozzle 1b. This spark discharge induces a pilot arc between the electrode 1a and the nozzle 1b. The formation of the pilot arc creates a closed circuit path starting from the positive terminal of the constant current source 8 and passing through a resistor 12, the switch 10, the nozzle 1b, the pilot arc, the electrode 1a, and finally returning to the negative terminal of the constant current source 8, whereby a pilot current I.sub.P, begins to flow through this path. At this stage, the constant current source 8 is in a state where it can output maximum power, or in other words, it acts as a constant voltage source. The resistor 12 provides a drooping characteristic, whereby the pilot current I.sub.P settles to a steady state value at which the arc voltage is balanced with the characteristics of the constant current source 8.
Thus, an electrical path between the electrode 1a and the nozzle 1b is established by the pilot arc. Then, a part of the arc current begins to flow toward the workpiece 11, whereby a main arc 13 is created. When a current detector 14c detects the creation of the main arc 13, the switch 10, which is connected between the constant current source 8 and the nozzle 1b, is opened, whereby the current path including the nozzle 1b is disconnected, and the current will flow only via the path including the main arc 13 and the workpiece 11. Subsequently, a cutting current I.sub.M detected by the current detector 14c is compared to a predetermined value, and the cutting current I.sub.M is controlled so that it is maintained substantially constant at the predetermined value. When a cutting or other required operation is complete, a stop signal S.sub.P is applied to the constant current source 8 so as to terminate the supply of power to the main arc 13, whereby the main arc 13 is extinguished. In this way, a desired cutting operation is performed.
However, whenever a plasma arc cutting machine requires a greater current, various problems arise. That is, the increase in arc current gives rise to: an increase in electrode wear; degradation in the transition from a pilot arc to a main arc; and an increase in the noise level of the plasma arc cutting machine.
First, the problem of electrode wear will be described. As the electrode material of the electrode 1a in the plasma torch 1 is directly exposed to the hot main arc 13, it is common to embed the electrode material in a holder that is cooled by water. In a plasma arc cutting machine that utilizes gaseous oxygen as the working gas, the electrode material can be either hafnium or zirconium, which can form an oxide having a high melting point. However, even if cooling of the electrode 1a is performed, and/or if a high melting point material is used for the electrode 1a, it is impossible to avoid the electrode wear. The greater the arc current is, the more serious is the electrode wear.
The electrode wear also strongly depends on the number of times the arc has been started. For example, as represented by the solid line P1 in FIG. 8, when cutting is done by a conventional oxygen plasma arc machine with a high current such as 250 A, if the number of start-up operations is about fifty, the electrode can be used for only about 3 hours in total accumulated arc time. This means that if the current becomes as high as 250 A, then the life of the electrode becomes as short as only 3 hours. If the start-up operation is repeated 400 times wherein each start-up operation is accomplished within a few seconds, the electrode can be used for only about twenty minutes in accumulated total arc time.
Now, the problem with arc transition will be described. When a thin plate is to be cut by utilizing a plasma arc cutting machine designed for thick plates, there are two options: one is to directly perform the cutting of the thin plate with the normal high arc current; the other one is to switch the arc current to a lower value. In the former case, the cutting should be done at a high speed to avoid overheating. However, the quality of the high speed cutting is limited as it is dependent on the tracking accuracy of a robot or an XY table on which the torch is installed. If this limitation does not permit the achievement of the desired cutting accuracy, there is no choice but to employ the latter technique. In the latter technique, a smaller size nozzle with a smaller orifice diameter is used, and the cutting is performed at a low speed with a low arc current which matches the smaller nozzle size. However, if the nozzle is damaged by a pilot arc, degradation in the cutting quality occurs immediately. If the voltage difference between the nozzle and a workpiece to be cut is great enough, then a high current flows through the workpiece when a pilot arc reaches the workpiece, and it is possible to generate a main arc for a short time even if the nozzle is at a rather high position above the workpiece. However, as can be seen from FIG. 6, the voltage difference between the nozzle 1b and the workpiece 11 is determined by the product of the resistance 12 and the pilot current I.sub.P. If the pilot current I.sub.P is reduced to match the smaller nozzle diameter, then the voltage difference between the nozzle 1b and the workpiece 11 also decreases, which gives rise to difficulty in the transition from the pilot arc to the main arc.
One known technique to reduce the electrode wear is to reduce or stop the working gas flow around an electrode just before a main arc is stopped (for example, refer to U.S. Pat. No. 5,070,227). However, a certain time delay in controlling the working gas flow is inevitable. In contrast, an electrical current can be controlled quickly. Even if a valve could be opened or closed instantaneously, the gas flow through the space between the nozzle and the electrode cannot change instantaneously due to the residual gas pressure in the portion of the gas supply pipe downstream of the valve. Therefore, after the cutting is complete, it is necessary to still maintain the arc until the gas flow reaches a low level at which the electrode wear is effectively reduced. As a result, after the cutting is complete, an additional portion of the workpiece will be cut wastefully. If the valve is disposed near the plasma torch, then the time required to reduce the gas flow will become short. However, this arrangement will give rise to difficulty in moving the plasma torch during its operation, and/or difficulty in installing the plasma torch on an XY table or on a robot.
In another known technique (for example, refer to Japanese Patent No. 47-30496) to solve the problems with the arc transition, there is provided a gas supply control unit, comprising a flow regulating valve and a flow stop valve disposed in parallel, between a gas source and a plasma torch. A gas, such as nitrogen, having a large heat capacity compared to argon gas and which can also serve as cooling gas, is used as the working gas. A small flow rate of the working gas is supplied for a pilot arc, and the flow rate is increased to a sufficiently high level when a transition to a main arc is performed. According to this arrangement, the start-up of the pilot arc and the transition to a main arc can be performed easily. While this technique can improve the reliability in the start-up of a nitrogen plasma arc, the problem with electrode wear still remains unsolved for oxygen plasma. In this technique, the pilot current should be greater than a certain value in order to achieve the transition to the main arc. This increase in the pilot current will result in greater electrode wear for the oxygen plasma.
Noise is another problem in the prior art. If the thickness of a plate to be cut increases, it is necessary to increase the output current. However, an increase in the output current also increases the noise arising from the plasma. One known technique to reduce the noise is to immerse a workpiece in water, and perform the cutting in water (for example, refer to U.S. Pat. No. 4,816,637). In this technique, while the noise can be reduced, the workpiece can become corroded, and a large scale treatment associated with water is required.